


Aurora Borealis (Last Light in the Dark)

by ohjustdisarmalready



Category: Undertale (Video Game)
Genre: Determination (Undertale), Gen, Magic, The Void, playing with the FUN mechanic, spacetime shenanigans
Language: English
Status: Completed
Published: 2020-03-21
Updated: 2020-03-21
Packaged: 2021-02-28 19:47:32
Rating: Teen And Up Audiences
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,683
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/23252692
Author URL: https://archiveofourown.org/users/ohjustdisarmalready/pseuds/ohjustdisarmalready
Summary: Shortcuts are an exact science.
Relationships: Alphys/Undyne (Undertale), Frisk & Sans (Undertale), Frisk (Undertale) & Everyone, Papyrus & Sans (Undertale), Papyrus & Undyne (Undertale), W. D. Gaster & Papyrus & Sans
Comments: 22
Kudos: 62





	Aurora Borealis (Last Light in the Dark)

**Author's Note:**

> Tbh this was supposed to be about something else entirely. I spent so long figuring out how to get characters where I needed them and keep them there that I thought, hey, what if Shortcut Broke due to a high FUN level (FUN being the mechanic you can alter in the game code that will cause Gaster Followers to appear, or Sans to prank call you). Something something void unstable, takes too much magic keeping everyone alive to shortcut back, etc. I was writing this more or less as a warm-up, but I figured someone out there would probably get a kick out of it, so I'm posting it here.
> 
> Also, I was too lazy to enter it into my "ideas" doc, so I had to write it. It's not an idea if it's already published.

Shortcuts are an exact science. They move through the Void—an infinitely-collapsing nothing dimension that takes up no space and time, but at the same time is uniform all across whenever it is accessed. The Void is a constant, while Sans and anyone following him and the starting point and the destination and a couple other environmental considerations are the variables. Most of the magic cost of a shortcut comes of enforcing regular six-dimensional reality onto the Void long enough to use it as a path to a regular six-dimensional destination.

If it’s just Sans taking a shortcut, he can cut out most of the cost—a relative second or two in the void won’t remove him from reality like it would most folks.

Sometimes he remembers why that is, sometimes he doesn’t. But he always remembers that the Void will hesitate to destroy him. The Void is as safe for Sans as…well, a relatively safe space in a dimension of infinite entropic constants. If he’s gonna have trouble with a shortcut, it’s gonna be in the world of variables, in himself or where he pops out or what he did wrong in his calculations.

Sans is very fast and very accurate in his calculations. He hasn’t shortcut himself into a staircase or table since…he can’t remember when. There was one time in particular, he thinks…

He’s pretty sure it’d scared…someone…Papyrus(?) so bad that he’d gotten a strongly-worded lecture about respecting the seriousness of his studies and not using highly advanced, theoretical magic to save ten seconds of walking. And the dangers of the Void, which always makes him snort when he thinks about it.

Come to think of it, he’s pretty sure he remembers Pap also being there for that conversation, saying he’s sure Sans knew what he was doing, and Sans wouldn’t endanger himself carelessly, so it must have just been a mistake _, right, Sans?_

That had actually made him feel worse than the lecture.

And then Papyrus had taken him quietly aside later, and asking him _Could You Have Fallen Down If You Did A Mistake?_ in the way that meant _Tell Me You Care More About Your Life Than This, I Know You Are Better Than This, I Am Already Impressed By Your Accomplishments And Wish You Wouldn’t Show Off If It Could Hurt You._ If Sans recalls correctly, he’d said something like…

“nah, paps, i was just—i was just being dumb, wasn’t thinking, forgot we moved that table, didn’t even hurt, right? just felt weird for a bit, it’s, uh, um.” He’d let out a breath and hung his head, unable to look at his brother’s increasing distress. “sorry. i shouldn’t have been showing off. i was just…yeah. that was stupid of me.”

He’d just been having fun with having a part of his studies he could show off to Papyrus and ****** without having to practically write a thesis to explain what it was and why it was kinda cool. Theory is great—Sans loves theory—but it was fun to have a practical application, and a useful one at that. He’d just gotten kinda carried away.

He’d thought he was being all responsible, owning up to what he did wrong (at least to Papyrus; he’d taken his brother’s out in front of ******) and apologizing for it, but Paps had seen right through him.

“That Was Actually Pretty Cool, Though,” Papyrus had said. Sans had perked up entirely, forgoing scuffing his shoes on the floor in favor of basking in his brother’s admiration.

“right? isn’t it? i can teleport!” he’d said. “i mean. maybe i should have…tested it out a bit more. it kind of _was_ an ‘irrational and dangerous misuse of untested theoretical physics,’ and all. lab safety is cool. that was…kinda not that.”

Not to mention how the shortcut that had taken him halfway through a table in the lab hadn’t been his first shortcut—he’d shortcut straight to Papyrus once he had a decent idea of how to balance out his equations for it, and spent most of the afternoon tweaking his technique and teleporting circles around his brother. That had been the opposite of lab safety. Especially when he left the lab to do it.

Somehow, Sans had figured he was just _smart enough_ that nothing would go wrong. “You’re never too good to make mistakes” is a lesson he’d learned later.

“I Was Not Really Encouraging Good Science Habits, Either,” Papyrus had admitted at the time. “It’s Very Cool That You Have Discovered This Neat Way To Use Your Magic! But! In The Future! Maybe We Can Make A Corner Of The Room That We Don’t Put Things In? And Then? You Use Your Cool New Magic To Go There? And Not In A? Table???”

Sans hadn’t said it then, because he’d had some sort of hang-up where he didn’t say it out loud, but that had been one of the many times that Papyrus had been _so cool_. Even when Sans had been, in hindsight, kind of insufferably proud of his scientific leanings, Papyrus always let him know that he thought what Sans was doing was cool and impressive, and that he was proud of his brother’s accomplishments.

Sans can remember that incident most of the time, even though it doesn’t really make sense with the memories he also has where he never told Paps about his interest in science, and told him he was going to dentistry school instead. The dentistry story is the one he _always_ remembers, though, so that must have…ugh. It always gives him a headache to remember this stuff.

Not worth the effort. He’s never gonna get that past back anyway, so why bother sorting out what really happened? Better to take what he has now: the coolest brother in the world, some pretty good friends, some pretty bad jokes, the _sky_. The _Surface_.

He’d never been as impressed by the idea of the Surface as other monsters seemed to be—seemed like it couldn’t be that great when Sans already had everything he needed underground with him. Not worth risking that for some world they’d been cut off from for thousands of years, right?

Stars, he’s never been happier to be wrong.

What he has now is everything he’d ever had underground, plus an ever-widening world of friends and family; sights to see, foods to try, things to avoid doing. He sure is glad he did absolutely nothing while Frisk did all the work of getting the Barrier down.

At least, he assumes he did nothing. Frisk won’t say what happened, and no one else remembers.

There are a lot of things Frisk won’t say. And it’s not just because they don’t talk much.

Doesn’t stop them from being a curious kid, though.

Today, Sans is relaxing, enjoying his escape from Alphys’s “SELF-ESTEEM TRAINING!!!” with Papyrus and Undyne, when Frisk trots up to him and stops, staring down at him insistently.

“‘sup, buddy?” he asks. “you find a cool rock somewhere?”

Frisk sometimes brings artifacts back from their adventures—clothes, tools, toys; almost always human artifacts that they seem to pick up from where no human had any business being in the first place.

Doesn’t look like that’s what’s going on now, though. They just have their stick with them, and the locket they picked up somewhere Underground. They’re clutching their shirt hem.

“…home?” they ask, craning their head to look behind them. They scan the surrounding buildings and trees at the edge of town before they check on Paps and crew.

Sans scans the area, too. Frisk hadn’t gone far—he’d been keeping an eye socket on them, or at least their SOUL; they hadn’t been out of his sight for more than a few minutes.

Then again, they do get nervous when they haven’t seen Toriel or Asgore for a while, even after years have passed and their new parents have failed to abandon them. Maybe they’re just antsy?

Undyne suplexes a dumpster unexpectedly, and Frisk whips around to face the noise, one hand outstretched like they would shield Sans from Undyne’s reign of terror against his fellow piles of garbage.

A tense moment passes. Frisk stares at Undyne, who is attempting to suplex Alphys, who is being carried by Papyrus. It looks like they’re playing some sort of keep-away game.

“pretty jumpy there, buddy,” Sans notes. Frisk glances at him, before going back to scanning their (still perfectly normal) surroundings. “makes a guy wonder. what’s rattling your bones?”

Not even a huff. Frisk just shrugs uncomfortably.

“come on. you can trust me, right? tell your buddy sans what’s wrong.” He winks his right eye.

Frisk squirms. They’ll break in a second, Sans is sure of it. Just a little more pressure…

Frisk’s eyes dart away, focusing intently on nothing, and their body twists to follow their gaze. They trot towards it two paces without even seeming to realize it, before their movement is arrested.

They reach up to clutch at something—hard to say what, from Sans’s vantage point on the ground behind them. Their collar, maybe?

They back up a half pace, making the tiniest conflicted noise. They shake their head.

“uh. frisk?” Sans asks.

Frisk glances at him, turning to keep both him and the point of nothing that’s caught their attention in their field of view.

Sans doesn’t know a lot about human eyesight, but if he doesn’t miss his guess, they’re focusing far away. Are they looking at the end of town or past it?

He can’t see anything there, but Frisk sometimes knows things he doesn’t. He’ll have to come back here later and see if he can rustle anything up.

“ _Home_ ,” Frisk insists. “We shouldn’t be here.”

They reach down towards Sans, and Sans forgoes jokes for a moment to let them pull him up. Frisk doesn’t seem like they’d take the old whoopee cushion joke too well right now.

Normally that wouldn’t stop him, but he doesn’t want to be the shout that causes the avalanche. Frisk is pretty fearless normally. Whatever’s spooked them could be pretty…spooky.

They don’t let go of Sans once he’s on his feet, choosing instead of grab his sleeve and tug insistently. He lets them propel him along, taking an arcing path towards the others.

If the kid’s keeping an even radius from whatever’s scared them, that puts it past a stand of trees. Nothing he can see right now, and they shouldn’t be able to, either.

Sans stares at the back of Frisk’s head _. what’s going on here, kiddo?_

Frisk doesn’t seem to notice, focused between Papyrus and the trees, until they glance back at him again.

“Do you…” they start. Then they shake their head, apparently deciding that whatever it is, he doesn’t.

“oh, yeah. all the time,” he says vaguely.

“No, no. Nothing. Nothing,” Frisk says. “It’s—did you ever—work with—nothing. Never mind.”

“i’ve had a few jobs, kiddo, i’ve worked with a lot of people,” Sans comments. “maybe a name would jog my memory?”

It’s not a joke, but Frisk snorts, only a little off-beat.

“No, it wouldn’t. Never mind. You should go home. Bring everyone. Game night.” They tug Sans forward and push him the last few feet, to Papyrus, Undyne and Alphys.

The training crew has politely paused midway through a game of catch with Alphys as the ball. Undyne sets her down by the head as Sans and Frisk approach.

“What’s up? Here to join in our EPIC TRAINING SESH?!?” she shouts.

The force of her passion generates a gust of wind that goes right through Sans. Frisk, on the other hand, has to cling to his hoodie to avoid being pushed back, at least until Papyrus’s blue magic anchors them more firmly to the ground.

“Brother! I Am Glad You Have Finally Chosen To Join Us, Instead Of Sitting Under A Tree And Being Lazy All Day! I Am…Surprised You Have Finally Chosen To Join Us! Shocked, In Fact!” Papyrus says. His eye sockets narrow. “…This Wouldn’t Happen To Be A Jape That You Are Concocting With Frisk, Is It? Human! I Have Told You Not To Succumb To My Brother’s Lazy Trickery! You Should Join Us In Training Just To Block Out His Tomfoolering Influence!”

Sans cuts in before he can get on a tangent—normally he has fun egging his brother on, but Frisk just might try pushing them all home if they dawdle too long, and funny as that would be, Sans doesn’t want to know what lengths Frisk would go to to get them all settled into a relative “safe” area.

“frisk just wanted to head home a bit early. too much fun out here, figured we’d ask who’s up for game night,” Sans says.

Frisk lurches and looks at him strangely, even though he’d pretty much just said what they had. Unless “too much fun” means something different to humans. Sans should remember to look that up some time after he stalks Frisk’s encounter with whatever’s out there.

It’s not stalking if you have parental consent, right? It’s, uh…temporary…guardianship. That promise he made to Tori all those years ago counts for that, right?

Welp. Too late to be quibbling over moralisms, anyway. Sans is in this deep, and it’s not like the kid is gonna let anyone else know what’s going on.

Papyrus hesitates, a bit thrown by the sudden plan change—they have a game night already scheduled for tomorrow, tonight was supposed to be relatively free so he could prepare a Meal of Epic Proportions for it—but Alphys is perceptive as Sans knows her to be.

After being placed on the ground, she’d gotten her bearings and read some part of Frisk’s frantic discomfort off of their posture. Their face has resumed a blank expression, but their free hand is clutching their locket, and they’re standing half-hidden behind Sans closer than usual.

Or, from the perspective to the point they’re avoiding, Sans is half-hidden behind them, with Papyrus and Undyne grouped behind them both.

While Papyrus is talking, Alphys makes a subtle gesture to Sans _—??—_ and he shrugs one shoulder and tilts his head back— _dunno, but i’m going along with it_.

This is not the first nor the last time Alphys will be frustrated by Sans’s staunch refusal to get overtly involved in a puzzling situation, but she does trust him, and she knows him well enough to know he’ll be doing his own investigation later. She pipes up in support.

“I-I think th-” The weight of everyone’s attention makes her flinch, but not as bad as she was during the worst of the amalgamates mess. She steadies after a moment. “I think that that would be f-fun. We c-could bring over something to…”

Frisk’s grip on Sans’s sweater, still there, tightens a fraction—again on the word ‘fun.’ Their stick shifts against his back. Where they’re holding it it’s sort of like a weird seat belt, or some kind of really ineffective shield.

Frankly, if anything were to get through the human at his back, there’s not a lot that would make an effective shield, except maybe magic. Given that Frisk only has DETERMINATION, a stick is as good as anything else, Sans supposes.

Sans shifts slightly, not even enough to be adjusting his weight, and moves his hand a bit. _nope._ Hopefully Alphys’ll get his meaning—Frisk said they should go to Sans and Paps’ house, and they seem to want to keep everyone together. Whatever is suggested needs to fill both of those conditions.

Jeez, this is really straining his teamwork muscles. It’s been forever since he and Alphys have had to be this in sync. Last time they worked together on something sensitive was…uh…

“Actually, I think! We left off right before the series finale of Kissy Kissy Mer-mew! And even if it’s not! As good as the original! Th-that’s still at your house! So we can watch that! And try making popcorn again!” Alphys quickly course-corrects, and Frisk relaxes. Sans nods, and Alphys relaxes, too.

Oh, he is really not getting out of this without explaining something to her later. He owes her several favors for this one.

Well, maybe one less, with the popcorn suggestion. She has essentially sacrificed his kitchen.

“Good,” Frisk says, pushing at Sans again.

Undyne and Papyrus, watching this interplay like a particularly subtle ping-pong match, decide it’s not worth questioning. Except…

“Hey, isn’t you guys’ house that way…?” Undyne points back to their house, snowy roof clearly visible, a hundred or so feet from the Dreaded Copse of Evil Trees.

“don’t worry.” Sans winks. “i know a shortcut.”

Frisk jerks his shoulder back, and-

Sans takes a shortcut. Taking four other people with him is a bit more costly, magically speaking, than he usually prefers; but whatever’s intimidating Frisk will probably be disconcerted by the Void, right? Home field advantage.

He makes up the calculations for his shortcut on the fly, like he always does. It’s a short trip spatially, and he has a good grip on everyone’s magical signatures to pull them with him. It should be an easy trip. Something he’s so accustomed to at this point that it’s barely math, more intuition. He knows what the hit to his magical reserves will feel like, he’ll be a little hungry and a little sleepy; he knows where he’s going; he knows he’s going to dump Frisk in a snowbank because they could use a laugh.

It should be totally forgettable.

Instead, the Void bucks and writhes, around him and through him like a living thing, like drowning or being crushed or ripped apart into infinitude, and in that endless instant of travel, he grasps every atom, every scrap of energy that makes himself up, and desperately scrabbles for something to keep it together. What shape is he supposed to fit together in? What _is_ he? He’s—

Magic. Magic, that’s what he’s doing. He’s using magic to travel, and he can use it to hold things, move things, can’t he? A stray photon attempts to escape form him and he grabs everything near him, everything he can feel, with blue magic, drags it to him—

Sans. He’s Sans. He’s Sans the skeleton, whole and in the corrupted void—it acts almost like a physical space, right now, flashing lights that he experiences as numbers and words, FUN on a little graph that spikes up and up. If he looked deeply into it, he’s certain that he would understand what went wrong, what happened, how it all works; the void could show him so _many_ things he doesn’t understand.

Instead, he reaches out with his blue magic. _someone, anyone._

Falling apart in the same way he was, he can feel his friends. He grasps for Papyrus first; he’s similarly resilient against the Void normally, but Sans isn’t sure “normally” applies right now.

It’s the work of an hour, the work of an instant to crush Papyrus’s magic back into a workable SOUL, and in that infinite moment he gathers his brother’s flaking dust and forces magic and light into it. Intense blue gravity fuses dissipating particles back into one being, and he reaches for Alphys. She’s the least DETERMINED, therefore, most vulnerable to harm.

His magic falters. It’s been a long time since it’s had this kind of workout. He grits his teeth and pushes harder. Time doesn’t work in the Void, so technically, he hasn’t spent any of that magic yet, right? He can use it simultaneously with himself. He’s only experiencing this in linear time because his mind is trying to make sense of what’s happening to him. As long as he doesn’t die or anything, he can reuse the same magic and the same moment. Right?

Sans is clumsier this time as he reaches blindly, messily towards Alphys’s relative direction. Her own magic should keep her alive as long as she’s not actively disappearing into the Void, right? He pushes her magic and dust into a general shape with a _crack!_ and moves on.

Oh. That crack might have been him, actually. Ouch.

…so, turns out he _does_ still have limited magic in nonlinear time.

Undyne isn’t falling apart so much as getting…gooey. Her DETERMINATION is trying to keep her together. Good, he just needs to get her out of the Void and hope it’s not too little, too late.

With effort, Sans searches for the other end of his shortcut, the inevitable finish to the calculation he’d based their path on, but…the Void is twisting, jerking around him—he can’t find it without looking through the Void, and once he looks, he’ll be lost.

Sans blindly tears a hole with a SOUL-deep _tug_ that tell him he’s reached the end of his energy. Weakly, already falling back into physical reality, he turns back to find Frisk—

\--who’s watching him, reaching with wide, wet eyes, as he and the others are saved, just a touch too far away to reach them.

Sans, out of magic entirely, reaches his physical hand back towards them, knowing already how futile it will be. Frisk’s mouth sets as they see something in his expression. He’s able to see an echo of their immediate future, how their hand will drop, how it will be taken up be an off-color afterimage of them, how both humans—human-shaped beings—will hold on to one another as they watch the tear in the Void close without them. Abandoned.

And then that future vanishes like a puff of smoke that someone put a bullet through, as someone else’s blue magic scoops Frisk up, sending them crashing into Sans and then both of them to Papyrus.

Sans’s brother is so cool.

Sans is able to see through his darkening vision that they’re falling through a dizzying expanse of gray and white, or at least that’s how it looks to him, before he passes out cold.

Paps can stick the landing for them.

**Author's Note:**

> So yeah, this was supposed to be setup for a longer oneshot, but tbh I feel like it's fine as it is??
> 
> Please let me know what you liked, if you liked it! I'm still trying to get a feel for these characters and how to write them together.


End file.
